DESCRIPTION (Verbatim from applicant's abstract): The Drosophila compound eye develops from the imaginal disc epithelium. Differentiation of a disc primordium into an adult eye involves dynamic cell-cell interactions and successive cell fate decisions. Our long-term objective is to understand the molecular basis of apical cell-cell contacts during retinal differentiation. Discs-lost (Dlt), a novel PDZ domain protein, is localized at the apical membranes. Dlt is essential for establishing apico-basal polarity of embryonic epithelia. Dlt is also required for proliferation and/or survival of imaginal disc cells. In the developing eye, Dlt appears to be crucial for specification of pigment cell fates and formation of retinal floor to support photoreceptor clusters within the retinal epithelium. Dlt protein contains four PDZ domains involved in protein-protein interactions. This suggests that Dlt may play multiple roles during eye development by interacting with specific protein partners. To study the role or PDZ domains of Dlt and interacting proteins, four specific aims are proposed. First, transgenic flies containing a series of truncated Dlt constructs will be generated. Mutant constructs will be tested in vivo to determine whether specific PDZ domains are required for disc growth and/or apical membrane localization of Dlt. Second, since loss of Dlt appears to cause misrouting of primary pigment cells, clonal analysis will be performed to determine which cell types need to express Dlt for correct specification of primary pigment cells. Third, interaction between Dlt and Delta will be examined at biochemical, genetic and immunocytochemical levels to confirm our preliminary evidence that Dlt interacts genetically with Delta, a membrane-bound ligand for Notch. Fourth, Dlt interacts with intracellular domain of Crumbs (Crbintra). Overexpression of Crbintra and loss of Dlt results in similar defects in the eye, suggesting that Crbintra acts as a dominant-negative factor for Dlt. The eye phenotype caused by Crbintra will be used as an efficient genetic system to find new mutations affecting Dlt and Crb function in the eye. Vertebrate protein sequences similar to Dlt have been found. These studies on Dlt and Notch signaling proteins will provide useful insights into the function of vertebrate homologs in cell-cell interaction and signaling.